Would it be incorrect to say — Evde bir hırsız var?
Nope, and this is already accepted :)
Why not hırsız evde var
You are talking about the existence of the "thief" and not of the "at home." For this reason, "hırsız" must go directly before "var" (var is used to talk about existence)
hırsız evde is right mi ?
"Hırsız evde" is a fine sentence :)